Diffusion induced recrystallization in alumina

When Fe2O3 was substantially dissolved into an (0001) alumina single crystal through a vapor phase at 1500 °C, recrystallization of the single crystal occurred. TEM observation showed that the nucleation of the recrystallized grains occurred through the formation of very small angle wall boundaries via a rearrangement of the misfit dislocations in the surface diffusion layer and their subsequent transformation into high angle grain boundaries. The dislocation wall mechanism appears, therefore, to be the nucleation mechanism of diffusion-induced recrystallization in single crystal alumina. However, an abrupt change in Fe2O3 concentration was observed across the boundary between the single crystal and the growing recrystallized grains, indicating that the growth of the recrystallized grains occurred by diffusion-induced grain-boundary migration. During extended annealing, the division of large recrystallized grains into small ones (repeated recrystallization) often occurred. The observed repeated recrystallization was attributed to the redistribution of dislocations formed in the grains where the solute concentration was non-uniform.